|M.Sc Student||Shlenkevitch Dmitry|
|Subject||CMOS-SOI-MEMS Transistors for Gas Sensing|
|Department||Department of Electrical and Computer Engineering||Supervisor||PROFESSOR EMERITUS Yael Nemirovsky|
|Full Thesis text|
In recent years, the need for mobile, low-cost and low-power gas sensors increased. Such gas sensors are needed for food supply chains to reduce food losses, safety at homes and cars, well-being of people and industrial process control.
Recently, novel uncooled thermal sensors based on CMOS-SOI technology, are pursued by the group of Prof. Nemirovsky. The sensor, TMOS, is based on a suspended micro-machined transistor fabricated in standard CMOS-SOI process and released by dry etching. The thermally isolated transistor, operating at subthreshold, converts small temperature changes to electrical signal since the transistor I-V characteristics are strongly dependent upon temperature.
This research focuses on a new generation of combustion-type gas sensors, GMOS, which are based on TMOS. The study emphasizes the design considerations and fabrication technology, the goal being to develop a miniature, cheap, sensitive, reliable and selective sensor that will be suitable for mass production.
The targets to reach the goal are: 1) GMOS performance modeling; 2) reliable catalytic layer deposition technology development; 3) experimental demonstration of the sensitivity, selectivity and reliability; and 4) methodology development of gas sensing in mixtures.
GMOS principle relies on a combustion reaction of an analyte gas occurring on a catalytic layer. The exothermic reaction releases heat to the ambient and increases the sensing transistor’s temperature. This change induces the transistor’s current-voltage characteristics change indicating gas presence. GMOS performance model was proposed and verified experimentally. The sensor’s key features are high sensitivity and selectivity. The TMOS high sensitivity to temperature changes provides GMOS sensitivity. The selectivity relies on the ignition temperature (T*) of combustion reaction. The T* is an inherent property of a catalyst and analyte gas combination, that enables detecting gas in mixtures.
The standard CMOS-SOI technology with MEMS backend processing enables manufacturing miniaturized sensors with excellent reliability. The transistor serves as a sensing element and promises a stable voltage temperature sensitivity. The sensor’s reliability remains limited by the stability of the catalytic layer. This work emphasizes the chemical aspects, catalytic layer deposition and treatment methods for GMOS gas sensor.
The catalytic layers were deposited using magnetron sputtering and inkjet printing technologies. These deposition techniques are suitable for wafer level processing and provide low-cost manufacturing. Both methods are compared in terms of gas sensitivity, where Pt nanoparticle catalyst shows superior performance over the sputtered porous Pt layer, and the signal can be detected below 1 ppm of ethanol. The selectivity is demonstrated for ethanol-acetone and ethylene-ethanol gas mixtures with a good accuracy. An extremely high sensitivity (up to 40 mV/ppm) is obtained for ethylene on Pt nanoparticle catalyst, showing a great potential for sub-ppm ethylene detection. This can lead to a breakthrough in ethylene monitoring in the fruits and vegetables food supply chain and reduce food losses.
To sum up, novel GMOS gas sensor’s key features such as miniature size, low-cost production, low-power consumption, fast response, high sensitivity and selectivity are discussed in this work. These advantages of GMOS are essential for future wearable, IoT and smartphone applications.